Wind turbines are commonly used since centuries to convert wind into work. A wind turbine typically is an impulse turbine. The turbine changes the direction of flow of the wind and the resulting impulse spins the turbine and leaves the wind with diminished kinetic energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or pumping water, the device is usually called a windmill or wind pump. Developed for over a millennium, today's wind turbines are manufactured in a range of vertical and horizontal axle types. The most common and oldest ones are the horizontal axle turbines, wherein the axles are positioned in line with the direction of the wind (i.e. the direction in which the wind blows). The smallest turbines are used for applications such as battery charging or auxiliary power on sailing boats, while large grid-connected arrays of turbines are becoming an increasingly large source of commercial electric power.
One of the developments of the last decades is to devise wind turbines that are suitable for low wind velocities (typically below 10 m/s, about 5 Beaufort). In particular at lower heights, above land and in the presence of buildings, wind velocity is often too low to economically extract energy from common wind turbines. Recent developments include the Darrieus (including giromill and cycloturbine) and Savonius wind turbines which may generate mechanical energy even at a wind velocity below 5 m/s (about 3 Beaufort). Unlike the Savonius wind turbine, the Darrieus is a lift-type turbine. Rather than collecting the wind in cups dragging the turbine around, a Darrieus uses lift forces generated by the wind hitting aerofoils to create rotation. With these turbines, the axles are positioned transverse to the wind (as opposed to “in line” with the wind) which has the additional advantage that the vanes do not need to be pointed to the wind. In most cases the axles are positioned vertically (which explains the commonly used acronym VAWT: vertical axle wind turbine), but they may also be positioned horizontally as long as the axle is transverse to the wind (TAWT—transverse axle wind turbine—would thus be a more correct acronym), typically substantially perpendicular to the direction in which the wind blows. These wind turbines however have several disadvantages. Firstly, the overall rate of conversion of kinetic into mechanical energy of these turbines might be improved. Secondly, in particular Darrieus wind turbines are not self-starting. Therefore a small powered motor is required to start off the rotation, and then when it has enough speed the wind passing across the aerofoils starts to generate torque and the rotor is driven around by the wind. An alternative constitution is the use of one or two small Savonius rotors that are mounted on the shaft of the Darrieus turbine to start rotation. These Savonius rotors however slow down the Darrieus turbine when it gets going.
From WO2009/153772 (Kirpitznikoff, filed 2008) a wind turbine as described in the preamble (Field of the invention section, here above) is known.
From WO2014/006075 a wind turbine as described in the preamble is known, apart from the one or more profiles on the inner surface of the lid. The vanes of this turbine are provided with multiple separate wind receptacles and means operable to force the receptacles to adopt either an open or a closed configuration independent from the direction of the wind. Advantages of this known wind turbine are that the energy conversion is considerable higher than with more conventional wind turbines, in particular at low wind speeds. Also, due to the fact that the receptacles may be forced to actively take a configuration against the forces of gravity and/or wind, makes that the turbine requires only a very low wind velocity to start turning around. Next to this, by being independent from the direction of the wind for adopting either of the two configurations, the turbine allows to achieve a very high maximum conversion of wind force into motion.
From CN 101737252A also a wind turbine according to the preamble is known. This wind turbine is used to start a Darrieus type of turbine. A disadvantage however is that like other VAWT's, the overall rate of energy conversion is relatively low.
U.S. Pat. No. 5,570,997 discloses a wind turbine having a vane provided with a wind receptacle that opens when the vane is traveling in the direction of the wind and closes when the vane is traveling against the direction of the wind. This way, maximum capacity of the wind receptacle is used when the wind force has to be converted into motion of the vane, while the receptacle hardly counteracts this conversion when the vane travels against the direction of the wind. The receptacle opens by action of the wind blowing over the vane and closes under gravity when this wind action decreases.